Molecular harpoon

Bilayer vesicles, especially the non-isotonic ( stressed ) kind or those which contain large amounts of dissolved cholesterol, can also be made to leak with the aid of molecular harpoons . These are membrane-disruptive surfactants comprised of a rigid, wedge-shaped hydrophobic unit attached to a hydrophilic chain, e.g. 22. It was discovered that just one harpoon molecule per ten... 

Naka, K., Sadownik, A., Regen, S. L. (1993). Molecular harpoons. Membrane-dismptive surfactants that can recognize osmotic stress in phospholipid bilayers, J. Am. Chem. Soc., 115 2278. 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

Figure 17. (a) Generic reaction path for charge transfer reactions with both channels of harpooning and electron transfer indicated. Molecular dynamics of the Bz/l2 bimolecular reaction is shown at the bottom, (b) Observed transient for the Bz/l2 reaction (I detection) and the associated changes in molecular structure. Note that we observe the two channels of the reaction, shown in (a), with different kinetic energies and rises of the I atom. 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

Harpoon mechanism Reaction sequence (thermal or photoinduced) between neutral molecular or atomic entities in which long-range electron transfer is followed by a considerable reduction of the distance between donor and acceptor sites as a result of the electrostatic attraction in the ion pair created. 

The extended set of locally excited conformations will produce the charge-separated (CS) set of states, whereas the folded set will generate the exciplex state. Finally, Coulombic-induced molecular folding (harpooning [57]) in the extended set of CS states will lead to increased production of the exciplex state. The dynamic competition between the various processes outlined in Figure 15 depends not only on the chain length and the redox properties of the donor and acceptor groups, but also on solvent polarity. 

After these two examples, it should not be thought that reactions described by the harpoon model are necessarily direct. This was the case with reactions 4 and 5 because they are very exoergic and the molecular negative ion is unstable. In contrast, in the almost thermoneutral reaction of Cs with NO2, NOj is a stable ion, and the charge-transfer complex Cs - NOj corresponds to a deep well along the reaction coordinate. Hence the reaction proceeds from a persistent complex, with a forward-backward symmetry in the angular distribution of the reaction product CsO [72]. 

The geometry of molecular anions generally differs from that of the neutral molecule. For instance, N20 is bent whereas the N2O molecule is linear. This is one of the reasons why the vertical electron affinity of N20 is negative. This is also the reason why bending excitation increases the electron affinity [EA) of the molecule. From Magee s equation (see Eq. 3), we know that the increase in EA results in a larger distance Rc for the harpoon. A larger cross-section is therefore expected. This effect has actually been observed semiquantitatively in the chemiluminescent reaction Ba -t- N2O BaO + N2 [125, 126]. The same effect was confirmed later in an experiment where the internal excitation of N2O was carefully prepared [127]. It... 

The model of electron transfer in gas-phase metal-molecule reactions can be extended to more complex systems such as the collisions of metastable rare gas atoms with molecules to produce negative molecular ions [306], In surface chemistry the harpoon model describes the forces between the reagents after the electron transfer has been applied to reactions of molecules with metal surfaces [120]. Another domain, involving the reaction of metal ions with complex systems could be interpreted in the framework of electron transfers in the porphyrin site of the heme within hemoglobin, addition of oxygen to the Fe " " results in an electron transfer from the metal to the oxygen. The dynamics of this attachement and of the photo-induced detachment could be viewed in that perspective. 

R. Brooks [42-44] the process of electron transfer for K to oriented t-butyl bromide is found strongly dependent on the orientation. Systems involving metal atoms are traditional favorites of molecular beam studies, particularly of stereodynamics. In recent experiments [45], with brute force oriented ICl, experimental determination was made of the cone of acceptance for reactivity (steric effect) in a "harpooning" reaction, Sr + ICl leading to electroiucally excited products detected via their chemiluminescence... 

P.R. Brooks, Molecular beam reaction of K with oriented CF3I. Evidence for harpooning , J. Chem. Phys. 50, 5031-5032 (1969). 

Hernandez-Trujillo and Bader studied the evolution of the electron densities of two separated atoms into an equilibrium molecular distribution, and considered a range of interactions from closed-shell with and without charge transfer, through polar-shared, to equally shared interactions. The harpoon mechanism operative in the formation of LiF was found to exert dramatic effects on the electron density and on the atomic and molecular properties. The virial, the Hellmann-Feynman and the Ehrenfest force theorems provided an imderstanding of the similarities and differences in the bonding. 

The cover illustration is by the late Israeli physical chemist and artist Jacob Wilf. Jacob was my friend and we had many scientific discussions. One result is that he has drawn several paintings depicting themes from Molecular Reaction Dynamics such as harpoon reactions, stereodynamics and cluster impact. The painting shown on the cover is titled surprisal analysis and was dedicated to me by the artist. The topic of surprisal analysis is discussed in Section 6.4.2. The works of Wilf on a variety of scientific themes were exhibited at the Israel National Academy of Sciences and other Institutions. Many paintings by the late Jacob Wilf can be seen at http //j acob. wilf org/... 

---